Surface contour measurement



Patented July 21,' 1953 SURFACE coNroUa MEASUREMENT Philip J. Herbst, Moorestown, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application June 28, 1949, Serial No. 101,816

10 Claims.

This invention relates to improvements in the art Vcf examining the surface of an object, and particularly to an improved method of and apparatus for obtaining an accurate measurement of surface contours or profiles.

Various methods have been proposed for obtaining an accurate indication of the surface contours of both uniform and irregularly shaped :objects to determine whether or not the contours of the objectsurface conform to a reference pattern, or to determine whether or not the dimen- .sions of the object -being examined ycome within permissible tolerances For example, it is common practice to caliper die models and the like to check the shape and measurements thereof for conformance with a pattern. Priorart methods of making such examinations are generally laborious and time consuming, require considerable manipulative or interpretive skill, and are not always as accurate as might be desired.

t is, accordingly, a principal object of the present invention to provide an improved method of and apparatus for simple and vaccurate measurement of the surface contours of an object.

Another object of the invention is to provide for surface contour measurement without Vphysical contact between the measuring device and the surface being examined. f v

A further object of the invention is the provision of surface contour measuring apparatus which will present an accurate picture of surface 4contours along a selected section through the object being examined.

Another object of the invention is to provide an improved method of and apparatus rfor checkface with a pencil of light, or by scanning an electrical image of the immersed object surface with an electron beam, an electrical signal .can be generated which will represent a surface contour taken along any desired plane through the lobil ect.

A more complete understanding of the invention can be had by 4reference to 'the following vdescription of rillustrative embodiments thereof,

, when considered in connection with Atheaccom- `panying drawings wherein:

Fig- 1 Shows schematically an apparatus for 2 making contoury measurements in accordance .with the invention,

Fig, 2 illustrates a modified form of the apparatus of Fig. ,1.

It is .well known that the intensity of a .beam of light will always decrease to a greater or less extent when passed through matter in the solid, liquid, or gaseous state, unless the medium is `perfectly transparent, and that the loss of intensity is due chiefly to absorption. In generaLvfor a homogeneous absorbing medium, the intensity of the light beam will decrease exponentially as a function of the thickness and absorbing characteristics of the medium (see e. g. `Fundamentals of `Physical Optics, Jenkins and Whit-e, pp. 2l-23). 1n accordance with my invention, these general principles are utilized in examining the Surface of an object to determine the contour or profile thereof.

Referring to Fig. l, there is shown in cross section a casting l0 which is to be compared With a model or pattern (not shown). The casting l0 is immersed in a bath of light-absorbing liquid I4 in an open tray i12 or other suitable container. The liquid IA can be any liquid which will partially attenuate light rays passing therethrough, such as a `colloidal `dispersion of graphite in oil or water, for example. The surface I Ga to be examined is preferably etched, or coated with a removable reiiecting lm such as white paint having a finely ground pigment, in order 4that the surface G9a will have substantially uniform light-reflecting characteristics. In the illustrative embodiment shown, the casting l0 is `supported in the tray I2 on knife-edge blocks i6 or similar supports which will establish a reference plane through the casting parallel to the surface of the liquid ifi.

In general, where thickness dimensions `are to be measured, the bottom surface of the `object will be taken as the reference plane as in the example shown, and the thickness dimensions will be obtained by measuring the distance between .points von the surface of the object and the surface of the liquid. However, in some cases a profile will be of interest with relation to some reference plane other than the bottom surface of the object, and in such case the object 4will be suitably supported to orient the desired referencre plane parallel to the surface of the liquid.

By illuminating the casting surface lila with .light passed through the liquid ifi, and measuring the amount of light reflected from portions line can be determined. For example, as shown in Fig. 1, the surface Illa of the casting I can be illuminated with a beam or pencil of light I8 generated at the fluorescent screen 22 of a cathode ray tube 26, and light reflected from the surface a of the casting can be picked up and measured by light sensitive elements such as two or more photoelectric tubes 4I] disposed above the container I2.

The intensity of the reflected light reaching the phototubes l0 will be a function of the light absorbing properties of the liquid III, and of the distance which the light has traveled through the liquid I4 in passing between the surface of the liquid and the surface lila of the casting. Consequently, the amount of light picked up by the phototubes di? will be a function of the distance that the light has traveled down and back through the liquid III for any given position of the beam I8, and, hence, will be a measure of the distance between the surface of the liquid I4. and the casting surface lila for that particular beam position. By correlating the output of the phototubes and the position of the beam IB, it is possible to obtain the prole of the casting I along the path followed by the beam I8.

Considering the apparatus of Fig. l in greater detail, the cathode ray tube 2i) is provided with the usual electron gun 29, and with horizontal and vertical beam deflection coils 25, 28. It will be understood that the terms horizontal and "vertical, as used herein, have no absolute significance, but are used only to indicate orthogonal deflection of the cathode ray beam. For simplicity, the D. C. operating potentials for the tube 29, and the details of the gun 22, have been omitted.

The tube 20 is disposed in a housing 39 which has a lens 3| mounted in one wall thereof, facing the tube screen 22, to direct the light beam I8 onto the surface lila. Although the container I2 is shown as being quite close to the housing 3|! because of space limitations in the drawing, it will be understood that the container l2 and housing Sil should be spaced apart, or the optical system suitably designed, so that the light beam I3 will always enter the liquid I@ approximately at right angles to the surface thereof.

In order to cause the beam I8 to scan the surface Ia, the horizontal deflecting coil 26 of the cathode ray tube 2e is connected to a dellecting generator 32 which will supply a sawtooth Waveshape current to the deiiecting coil 2B to deflect the cathode ray repeatedly across the screen 22.

In the embodiment of the invention presently being described, the particular contour to be examined can be selected by adjusting the vertical position of the cathode ray beam with a potentiometer 3G connected across a battery 3S. That is to say, a particular section through the casting Ill can be selected by adjusting the vertical position of the cathode ray beam (in a direction at right angles to the plane of the drawing).

The phototubes 4|) are supplied with operating voltage from a battery l2 through a load resistor Ml. The amount of current now through the load resistor llli will be a function of the amount of reflected light reaching the phototubes 4l) from the casting surface lila, and, hence, the load resistor Voltage will be a function of the distance which the beam I8 will travel through the liquid I4 in any given beam position.

A preferred type of indicator for displaying the information obtained from the phototubes comprises a cathode ray display tube 50. The display tube 5t contains an electron gun 52 and horizontal and vertical beam deection plates 54, 56, with the vertical plates being connected to the phototube load resistor 4:1, preferably through a D. C. amplifier 46, so that the vertical deection Voltage for the display tube 50 will be the profile information voltage obtained from the phototube circuit. The horizontal deection voltage for the display tube 55 is obtained from a horizontal deflection generator 58, and comprises a sawtooth waveshape Voltage timed to coincide with the sawtooth horizontal deilection current for the light beam generator tube 25. For example, the two sweep circuits 32, 53 can be coupled to a synchronizing signal generator Sil which will supply a signal to control or trigger both sweep circuits simultaneously.

As the cathode ray beams in the scanning and display tubes 2li and 50 move across the tube screens 22 and 51, respectively, the amount of reflected light picked up by the phototubes 4E will vary in accordance with the cross-sectional dimensions of the casting Ill. The vertical deflection voltage for the display tube 5E! will vary correspondingly, presenting a proiile view of the casting I I) on the display screen 5'I.

In accordance with the general principles of light absorption, the amount of reflected light picked up by the phototubes 40 will vary exponentially as a function of the distance between the liquid surface lila and the casting surface Illa. For this reason, the profile obtained on the screen 51 of the display tube 5S actually will be a logarithmic curve. Accordingly, the screen 51 of the display tube is preferably provided with logarithmically distributed scale lines S as an aid in interpretating the display.

In Fig. 2, there is shown an alternative appara.- tus for practicing the method of the present invention. In this case, a cathode ray camera tube, such as a so-called iconoscope, is utilized to convert an optical image of the immersed object into an electrical image, and the electrical image is scanned to obtain prole information.

In Fig. 2, the object i@ to be examined is immersed in a bath of light absorbing liquid |11 in a tray i2, as in the case of Fig. l. The object surface Illa is subjected to uniform illumination With light from a plurality of shielded lamp bulbs |40 or the like, and an optical image of the illuminated surface ma is focused by a lens 3| onto the mosaic screen |22 of a cathode ray camera tube |20 to form an electrical image on the screen |22. The liquid Ill has the same eifect as the liquid I4 discussed above in connection with Fig. 1. The camera tube i2@ also has the usual electron gun |24 and collector ring 25, and is provided With horizontal and vertical deflection coils In accordance with the usual mode of operation of a camera tube of the type shown, the electrical image on the mosaic screen |22 will comprise a pattern of electric charges distributed in a pattern corresponding to the light distribution in the optical image focussed on the screen |22. By scanning the screen |22 with the electron beam |23, any charge deficiency at each point scanned will be eliminated, and the overall electrical potential of the screen |22 will assume a succession of different values depending on the charge supplied thereto at each instant. With the screen |22 coupled to the vertical deflection plates 56 of a cathode ray display tube 5S, through an amplier 4S, a profile of the object ID being examined can be reproduced on the display screen 5'I as in the apparatus of Fig. 1.

connected through a switch E29 to a vertical' sweep generator ist which will furnish a sawtooth waveshape current to the vertical deflection coil in order to produce a multi-line or raster type scan of the screen 22. The vertical sweep generator i3d is intended to furnish a sawtooth waveshape current at a submultiple of the operating frequency of the horizontal sweep generator l32. Asshown, the vertical sweep generator {34 can be connected to the synchronization signal generator 63 through a frequency divider network 52, so that all three sweep generators 58, 32, i3@ will be in step; the two horizontal generators 5t, 32 operating at the same frequency, and the vertical generator I 34 operating at a submultiple thereof.

The scanning system shown in Fig. 2 is particularly useful for checking dimensions of an object ofv uniform cross section in accordance with the method of the invention. If the profiles of the object are identical at each cross section scanned, a single, clear cut line will appear on the display tube screen 5l, and the dimensions can be checked immediately. However, if any one or more of the profiles is slightly different than the others, the display screen presentation will be blurred or multi-line, depending on the amount of variation between profiles. In either event, if the proledisplay on the screen 5l shows one or more departures exceeding permissible tolerances, then the operator will know that the piece being examined is not acceptable, and can switch the vertical deflection coil t28 from the vertical sweep generato-r it@ to the vertical beam-position control potentiometer 3d to examine individual profiles of the object surface Ilia. and determine the points of profile deviation.

It will be understood that either the single line scanning system or the multi-linescanning system, or both, can be used interchangeably in the apparatus of Figs. l and 2.

It should be noted that apparatus similar to that shown either in Fig. i or in Fig. 2 can be arranged for automatic or remotecontroi operations. For example, in a rolling mill operation,

a finished metal bar or rail could be run through a liquid bath beneath a cathode ray light beam source of the type shown in Eig. 1, or beneath a camera tube of the type shown in Fig. 2, and the output of: the phototubes or of the camera tube, as the case may be,'could be utilized to control the operation of the mill through a system of. relays and the like, or to furnish a continuous picture of the cross-sectional prole of the metal sheet to an observer. K

Furthermore, it is not necessary that the light reflected from the object surface be converted directly into electrical signals. For example, the object to be examined could be illuminated in the manner shown in Fig. 2, and a photograph taken of the immersed object surface. Bytscanning the resulting photographic transparency with a light beam, and pick-ing up-the= light transmitted through the transparency, an electrical signal similar to that obtained in the apparatus of Fig. l would be made available.

Since these and other similar changes could be made in the apparatus shown for practicing the method of the present invention, the foregoing is to be construed as illustrative, and not in a limiting sense.

What is claimed is:

1. .A method of measuring surface contours of an object each contour taken along a single plane having a uniform light-reflecting surface, said method comprising immersing said object in a light-absorbing liquid with a reference plane through said object oriented parallel to the Vsurface of` said liquid, directing light through said liquid onto saidl reflecting surface, and measuring the amount of light reected through said liquid from a continuous series of aligned points on said reflecting surface asa measure of the distance between each said point and the surface of said liquid.

2. A method of measuring surface contours of an object -each contour taken along a single plane having a uniform light-reflecting surface, said method comprising immersing said object in a light-absorbing liquid with a reference plane through said object oriented parallel to the surface of said liquid', directing light through said liquid onto said reflecting surface of said object, picking up light reflected through said liquid from points on said reflecting surface along a selected cross section of said object, and measuring the amount of reflected light picked up from each of said points as a vmeasure of the distance between each said point and the surface of said liquid.

3. A method of measuring surface contours of an object each contour taken along a single plane having a uniform light-reflecting surface, said method comprising immersing the object to be examinedy in a light-absorbing liquid with a reference plane through said object oriented parallel to the surface of said liquid, scanning said reflecting surface of said immersed object along a single liney with a beam of light, picking up light reflected through said liquid from said object surface, and measuring the amount of light picked up at each point along the path followed by said scanning beam as a measure of the distance between each said point and the surface of* said liquid.

4. A method as set yforth in claim 3 including the steps of converting said picked up light into an electrical signal the intensity of which is a function of the amount of light reflected from each said point, and deecting an electron beam orthogonally with the deflection in one direction being synchronized with said light beam scanning and the deflection in another direction being controlled by said electrical signal so that the path followed by said electron beam will be a function of the contour of said reflecting surface along the path followed by said scanning beam.

5. A method of measuring a surface contour of a uniform light-reflecting Surface of an object, said method comprising immersing said object in a light-absorbing liquid, uniformly illuminating said object surface through said liquid to form an optical image of said Object Surface as viewed through said liquid, converting said optical image into an electrical image consisting of an electrical charge pattern having charge deficiencies distributed in a pattern corresponding to the pattern of light distribution in said optical image, scanning said 'electrical image with a first electron beam, converting into an electrical signal changes caused by said beam in said charge pattern of said electrical image, and deflecting a second electron beam orthogonally with the deflection in one direction beiner synchronized with said first electron beam scanning and the deflection in another direction being controlled by said electrical signal so that the path followed by said electron beam will be a function of the contour of said reflecting surface along the path followed by said scanning beam.

6. Apparatus for measuring surface contours of an object having a uniform light reflecting surface each contour being taken along a single plane, comprising a bath of light absorbing liquid, means to immerse said object in said liquid bath with a reference plane through said object oriented parallel to the surface of said liquid, a light source, means directing light from said source through said liquid onto said reflecting surface, means including a light sensitive element positioned to intercept light reflected from said reflecting surface through said liquid, said means also including a cathode ray tube having a fluorescent screen, first beam deflecting means coupled to said light sensitive element to deflect the cathode ray beam in said tube in accordance with the amount of reflected light intercepted by said light sensitive element, and second beam deflection means to deflect said beam with uniform velocity in a direction at right angles to beam deflection effected by said flrst beam deection means to measure the amount of light reflected through said liquid from adjacent points along said reflecting surface to determine the distance between the surface of said liquid and each point on said reflecting surface.

7. Apparatus for measuring surface contours of an object having a uniform light reflecting surface, said apparatus comprising a bath of light absorbing liquid, means to immerse said object in said liquid bath with a reference plane through said object oriented parallel to the Surface of said liquid, a light source, means directing light from said source through said liquid onto said reflecting surface, and means to measure the amount of light reflected through said liquid from adjacent points on a straight line along said reflecting surface to determine the distance between the surface of said liquid and each point on said reflecting surface, said last named means comprising a signal generator positioned to intercept light reflected from said reflecting surface through said liquid, a cathode ray tube having a fluorescent screen, beam deflecting means coupled to said signal generator to deflect the cathode ray beam in said tube in one coordinate and second beam deflection means to deflect said beam with uniform Velocity in a direction at right angles to beam deflection effected by said flrst named beam deflecting means.

8. Apparatus for measuring surface contours of an object having a uniform light reflecting surface, said apparatus comprising a bath of light absorbing liquid, means to immerse said object in said liquid bath with a reference plane through said object oriented parallel to the surface of said liquid, a light beam source comprising a llrst cathode ray tube having a fluorescent screen, means to direct a beam of light from said source through said liquid onto said reflecting surface, means to move said light beam acrOSS said reflecting surface along a predetermined scanning path, said last named means comprising a beam deflection system for deflecting the cathode ray beam in said tube orthogonally, and means to measure the amount of light reflected from points on said reflecting surface along the path followed by said light beam as a measure of the distance between each said point and the surface of said liquid, said measuring means including (1) a light sensitive element positioned to intercept light reflected from said reflecting surface and (2) a second cathode ray tube having a fluorescent screen, deflecting means coupling said light sensitive element to said second tube to deflect the cathode ray beam in said second tube in accordance with the amount of reflected light intercepted by said light sensitive element.

9. Apparatus for measuring surface contours of an object having a uniform light-reflecting surface, said apparatus comprising a bath of light-absorbing liquid, means to immerse said object in said liquid bath with a reference plane through said object oriented parallel to the surface of said liquid, light source means positioned to illuminate said reflecting surface uniformly through said liquid, a cathode ray camera tube having a mosaic screen adapted to convert 0D- tical images focussed thereon into electrical images, said tube being positioned to receive an optical image of said immersed object surface on said screen, a beam deflection circuit connected to said tube to control electron beam scanning therein whereby to alter the charge pattern in the electrical image on said Screen, and means connected to said screen and responsive to changes in said charge pattern to measure the amount of light in said optical image from point to point along the scanning path followed by said electron beam in said tube as a measure of the distance between the surface of said liquid and points on said reflecting surface corresponding to s'aid points in said electrical image.

10. Apparatus as defined in claim 9 wherein said measuring means comprises a cathode ray display tube having a fluorescent Screen, and an orthogonal beam deflection system including a first deflection circuit synchronized with said camera tube deflection circuit to control electron beam scanning in said second tube and a second deflection circuit connected to said mosaic screen to deflect the second tube beam in accordance With said changes in said charge pattern.

PHILIP J. HERBST.

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